Clostridium difficile, the etiological agent of pseudomembranous colitis in humans and animals, produces two toxins, designated toxin A and toxin B, that are cytotoxic for tissue-cultured mammalian cells. Toxin B is approximately 1,000-fold more cytotoxic than toxin A per mg of protein, however. In addition to its cytotoxicity, toxin A also possesses enterotoxin activity and causes a fluid response when injected into ligated rabbit intestinal loops.
While toxin A and toxin B can be quantitated by their activity against tissue culture cells, the greater activity against tissue culture cells of toxin B interferes with the detection of toxin A by this assay. As a consequence, the two toxins have to be separated before the assay to determine the cytotoxic titer of toxin A. The different activities of toxins A and B in the rabbit intestinal loop and suckling mouse assays indicate that these assays might be useful in detecting toxin A; however, both assays are tedious and not very sensitive. An enzyme-linked immunosorbent assay (ELISA) using affinity-purified antibody against toxin A has been developed which is said to be specific for toxin A and does not require an initial separation of toxins A and B. The sensitivity of the disclosed ELISA procedure for tissue culture-positive fecal specimens is only 59%, but more sensitive enzyme immunoassays for toxin A and toxin B, respectively, have been reported. Laughon et al, J. Infectious Diseases 149: 781-88 (1984). Aronsson et al also discloses ELISA's for either toxin A or toxin B. "Enzyme immunoassay for detection of Clostridium difficile toxins A and B in patients with antibiotic-associated diarrhoea and colitis," Eur. J. Clin. Microbiol. 4: 102-07 (1985).
Lyerly et al have compared monoclonal antibody, affinity-purified polyclonal antibody, and monospecific antiserum against toxin A by counterimmunoelectrophoresis, latex agglutination, and ELISA. J. Clin. Microbiol. 21: 12-14 (1985). But a single monoclonal antibody-based assay sensitive to both toxin A and toxin B, without the above-mentioned interference effect, has apparently not been reported. Indeed, the absence of reported polyclonal antibody cross-reactivity between the toxins suggested that a cross-reactive monoclonal antibody might not exist.